1. Field of the Invention
The present invention generally relates to a centrifugal pellet dryer of the type which utilizes a bladed lift rotor conveying moisture laden plastic pellets or other solid particles upwardly within a cylindrical screen with the centrifugal force imparted to the particles by rotation of the lift rotor causing the particles to engage the interior surface of the screen, and moisture on the particles to be discharged through the screen in a manner well known in the art. More specifically, the present invention relates to a deflector associated with the internal surface of the cylindrical screen.
2. Description of the Prior Art
Centrifugal pellet dryers are well known in the art for separating water or moisture from plastic pellets and other solid particles, such as a slurry of water and plastic pellets produced by underwater pelletizers. Centrifugal pellet dryers of the prior art include a vertically disposed outer housing, a cylindrical screen oriented in the housing and a driven bladed rotor positioned centrally in the screen. The rotor moves water laden pellets or other solid particles upwardly within the screen with centrifugal forces imparted to the particles by the rotor causing the particles to move radially outwardly into engagement with the screen for discharge of water through the screen. The dried particles are discharged from the upper end of the screen and housing, and water is discharged from the lower end of the housing. Centrifugal pellet dryers of this type are disclosed in U.S. Pat. Nos. 3,458,045; 4,565,015; 4,896,435; 5,265,347 and 6,138,375, commonly owned by the Assignee of this application. In the operation of such dryers, the pellets or other particles being moved vertically and radially by the bladed rotor engage the cylindrical screen with substantial velocity and usually bounce off the screen back toward the rotor for imparting further vertical and centrifugal forces to the particles as they are moved upwardly inside the screen.
However, with the advent of newer plastic materials which form softer pellets, and the making of very small pellets, or so-called micropellets, on underwater pelletizers, difficulties have been encountered in drying such pellets in known centrifugal dryers. Further, known centrifugal dryers have encountered difficulty in drying ground flake plastic materials which are formed from recycled soda bottles, milk containers and the like, as well as certain other plastic particles such as ground battery casings. More specifically, softer and smaller pellets and plastic flakes, as well as certain other plastic and similar particles, tend to collect and circulate in the clearance band between the outer edges of the rotor blades and the inner surface of the screen rather than bouncing around in the manner of harder and larger pellets or particles. This circular flow of the softer and smaller pellets and plastic flakes and particles along the inner surface of the screen is sometimes referred to as xe2x80x9cbandingxe2x80x9d and reduces product flow through the rotor area of the dryer and increases power requirements for maintaining rotational speed of the rotor. Further, it has been found that banding also reduces the efficiency of moisture separation from the solid particles, can cause high amperage surge requirements within the dryer, and reduces overall efficiency of the centrifugal dryer.
Accordingly, there is a need to improve existing centrifugal dryers in order to overcome this problem of banding when separating solid particles and water slurries, especially with soft and/or small pellets and certain plastic flakes and particles.
The present invention is used with a centrifugal pellet dryer of the vertical type having a vertical cylindrical screen associated with a vertical housing and a bladed rotor oriented in the cylindrical screen for conveying a slurry of water and polymer resin particles upwardly in the screen. Centrifugal forces imparted to the solid particles by the rotor cause the particles to impact the screen to discharge water outwardly through the screen, while the dried particles are discharged from an upper end of the screen and water is discharged from the lower end of the housing in a manner well known in this art. Cylindrical screens for centrifugal pellet dryers are typically made from several screen sections which are vertically aligned and interconnected together.
In order to overcome the problems of such centrifugal dryers when separating water from soft and/or small pellets or plastic flakes, and certain other plastic particles, the inside of the cylindrical screen is provided with a deflector in the form of one or more vertical elongated strips provided on the inside surface of the screen. The edge or side of the strip which faces the direction of rotation of the blades on the rotor, and thus the solid particles being circulated by those blades, is preferably inclined from a leading edge at the screen to a trailing edge so that particles moving in a rotational direction or circulating along the surface of the screen are deflected back toward the blades on the rotor. The vertical strip disrupts the circular flow of the particles, particularly softer and smaller pellets, plastic flakes, and certain other plastic particles, and directs the solid particles radially inward toward the rotor blades. This disruption of the circular flow improves particle flow through the rotor area of the dryer by aiding in the rotor""s vertical lift of the particles and by eliminating particles banding.
Further, the deflector of the present invention reduces the power required for rotating the rotor at a desired speed and eliminates shutdown of the dryer due to high amperage surges required to drive the rotor when the plastic particles are banding along the inner surface of the screen, which banding increases the load on the bladed rotor. In addition, the preferred deflector structure of the present invention also assists in stiffening the cylindrical configuration of the screen.
Preferably in accordance with the present invention, all of the cylindrical screen sections of the centrifugal dryer, except for the lowermost screen section, are provided with deflector strips mounted on or extending into the interior surface of each screen section. The deflector strips are preferably aligned vertically if mounted in more than one screen section. Further, one to four deflector strips are preferred for each section depending upon the size of the screen, with the deflector strips circumferentially spaced around the interior surface of the screen section when two or more are utilized. Small diameter cylindrical screens may require only a single deflector strip, whereas larger diameter cylindrical screens may require three or four, or more, circumferentially spaced deflector strips.
The deflector strips are disposed in the clearance band or annular space between the outer edges of the rotor blades and the interior surface of the screen. Usually, the outer edges of the rotor blades are spaced from the screen a distance of about xe2x85x9c inch to about xc2xd inch, and the deflector strips therefore have a thickness, or project, less than this clearance space between the screen and blade edges. A preferred thickness for the strips has been found to be about 0.10 inches to about 0.25 inches, and most preferably about 0.14 inches. A preferred width for the deflector strips has been found to be between about 0.50 inches and about 0.80 inches, but more in one embodiment. In the preferred embodiment, the preferred width is about 0.62 inches. The edge of each strip facing the direction of rotation of the bladed rotor is preferably beveled or inclined from a leading edge at the screen surface to a trailing edge at the inner surface of the strip to form an inclined surface. In one form, the inclined surface is preferably angled at about 45xc2x0, although other angles could be adopted. In another form, the inclined surface is preferably angled at about 70xc2x0. While not preferred, the deflector strips of the present invention could present a leading edge without any bevel or incline.
The deflection of the solid particles toward the rotor is especially significant when the particles being dried have a tendency to collect on and move along the inner surface of the screen rather than bouncing back toward the rotor as occurs when drying harder or large pellets and particles. The deflector strip interrupts the circular flow of the particles around the clearance band inside the screen, and the inclined surface deflects the particles back toward the rotor. The deflector strips therefore eliminate the tendency of the particles to flow in a band around the inner circumference of the screen, and cause the particles to move radially back toward the rotor blades thereby increasing product flow and efficiency of moisture separation and reducing power requirements and high amperage surge requirements which sometimes can result in temporary shutdown of the dryer.
The deflector strips can be attached to the cylindrical screen or sections by any convenient attaching mechanism. One preferred mechanism includes a mating mounting strip having approximately the same strip size and shape, which mounts to the exterior surface of the cylindrical screen or section. The deflector strip on the inside of the cylindrical screen and the mounting strip on the outside of the cylindrical screen can then be bolted together through the wall of the cylindrical screen. The bolt head is preferably positioned on the deflector strip side, and counter sunk to present a smooth surface toward the rotor. The mating interconnected deflector strips and mounting strips also serve to stiffen the cylindrical screen or screen sections. While such a mechanism is preferred for mounting the deflector strips vertically within the cylindrical screens, other mechanical and other type mechanisms will readily occur to those skilled in the art.
In another form of deflector strip in accordance with the present invention, the elongated deflector strip is in the general form of an angle iron with the angle between the two flanges being approximately 110xc2x0, rather than the standard 90xc2x0, or perpendicular, relationship. In this form, one of the flanges of the angle iron is clamped between the outwardly extending vertical side edge flanges which are bolted together in forming the cylindrical screen or screen section. In addition to being clamped, the flange is also preferably welded to one of the side edge flanges. The other flange of the deflector strip extends into the clearance band or annular space between the outer edges of the rotor blades and the interior surface of the screen to present an oblique inclined surface to the advancing particles circulating in the annular space. The angle Of the inclined surface is on the order of preferably 160xc2x0 from a plane tangent to the cylindrical screen or section inner surface.
It is therefore an object of the present invention to provide a deflector on the interior of a cylindrical screen of a centrifugal pellet dryer in the form of one or more vertical elongated deflector strips which deflect the solid particles moving along the interior surface of the screen back toward the rotor conveying the moisture laden particles upwardly in the dryer screen in order to increase particle flow and more efficiently separate moisture from the particles.
Another object of the present invention is to provide one or more vertical deflector strips circumferentially spaced around the interior surface of the screen with the number of deflector strips being varied depending upon the diameter of the screen, with there preferably being one to four deflector strips.
A further object of the present invention is to position one or more vertical deflector strips on the interior surface of a cylindrical screen in a centrifugal pellet dryer such that the edge or side of the strip facing the direction of rotation of the rotor is inclined with the inclined surface having a leading edge adjacent the screen in advance of the trailing edge adjacent the path of the rotor blades to form a surface which deflects the pellets or other solid particles toward the rotor as the particles move along the interior surface of the screen.
Still another object of the present invention is to position one or more vertical deflector strips on the interior surface of the cylindrical screen in a cylindrical pellet dryer in accordance with the preceding objects in which the inclined leading surface on each deflector strip interrupts the tendency of certain type plastic pellets and other particles to collect and circulate in the clearance band along the inner surface of the screen, thereby more efficiently enabling separation of moisture from the particles by causing them to move back toward the rotor for engagement with the rotor blades.
Still a further object of the present invention is to mount one or more deflector strips on the interior surface of the cylindrical screen in a cylindrical pellet dryer by means of a mounting strip having approximately the same strip size and shape which mounts to the exterior surface of the cylindrical screen opposite the deflector strip which then can be affixed together through the wall of the cylindrical screen by an appropriate bolt or other fastener connection.
Yet another object of the present invention is to provide a deflector strip in the general form of an angle iron which can be clamped between the outwardly extending vertical side edge flanges, and preferably welded to one flange, which are bolted together to form the cylindrical""screen so as to position the other flange on the interior surface of the cylindrical screen to present an oblique inclined angle to the solid particles which collect and circulate in the clearance band along the inner surface of the screen.
Yet a further object of this invention to be specifically enumerated herein is to provide a deflector for the cylindrical screen of centrifugal pellet dryers in accordance with the preceding objects and which will conform to conventional forms of manufacture, be of simple construction and easy to use so as to provide a device that will be economically feasible, long lasting and relatively trouble free in operation.
These together with other objects and advantages which will become subsequently apparent reside in the details of construction and operation as more fully hereinafter described and claimed, reference being had to the accompanying drawings forming a part hereof, wherein like numerals refer to like parts throughout.